For reasons of comfort and safety, today's vehicles are often provided with electrically heatable vehicle seats. The driver's seat, as well as the other seats in the vehicle, can be so arranged that they can be heated by means of heating elements in the form of electrically conducting wires which can be placed in each seat in the shape of a heating coil. Such a heating element is normally placed in the back-rest and the cushion of each seat when the seat is manufactured.
The heating element is furthermore connected to a current feeding unit which delivers current. In this manner, the heating element can be heated to a suitable temperature.
There is a requirement for each seat to have a carefully adjusted temperature on its surface, i.e. on that surface which is in contact with the person occupying the seat. For this purpose, the temperature of the heating element can be detected by means of a temperature sensor which is arranged in the seat, close to the heating element, and which is connected to a central control unit. Using the temperature sensor and the control unit, the temperature can be detected. The control unit also comprises current feeding circuits by means of which a certain current can be supplied to the heating element during a time period until a certain desired set value for the temperature is reached. The adjustment of this set value can be done either by means of fixed resistances or by means of an adjustable potentiometer which is adjusted by the occupant of the seat in question.
Using the above-mentioned control method, current can be supplied to the heating element until the central control unit indicates (through the use of said temperature sensor) that the set value has been reached. When this happens, the control unit will cut off the power feeding. This will cause the heating element to cool down gradually. When the heating element has cooled so that its temperature again is below the set value, the power feeding will be resumed. The temperature control will continue in this manner as long as the system is operative.
The control of the supply of current to the heating element can be provided in other ways, for example by means of a control unit which is integrated in the vehicle (for example in the form of a control unit which normally is used for controlling the operation of the vehicle's engine or for air condition control of the vehicle's compartment). To this end, such an external control unit can then be adapted so as to communicate with the above-mentioned heater control unit. Arrangements for such communication are previously known from the patent documents WO 98/01798 and WO 98/01059.
Furthermore, in today's vehicles, it is common to use one or several airbags for the protection of the occupants in the vehicle. In this regard, there is a need for detecting the presence or absence of an occupant in a seat which is associated with an airbag. In particular, there is a desire to detect the load acting upon the seat, in order to provide an indication of the presence of an occupant or an object in a seat. The result of such detection can be used for activating or deactivating various functions, for example said airbags. This is due to the fact that an airbag for a passenger in the vehicle can cause severe injuries if it is activated when, for example, a rearwards facing child-seat is positioned in front of an airbag, or when a passenger who is not using a safety-belt leans forward at the moment of activation of the airbag. The result of such detection can be used for activating other functions in a vehicle, in particular for providing headlight adjustments or a seat belt warning.
For this reason, it is previously known to equip a vehicle with a sensor arrangement which is arranged so as to provide an indication of whether an occupant is present in a seat (which in turn is associated with an airbag) or an indication of whether an auxiliary child seat is positioned on the seat. Also, the the orientation of the child seat or the type of child seat being used is preferably determined. In this manner, an airbag protection system in a vehicle can be deactivated if, for example, a child seat is placed on the seat in question. This constitutes an advantage, since the risk for injuries to the person sitting in the child seat due to an activated airbag can be eliminated.
An arrangement for detection of the presence of a person or an object in a vehicle seat is previously known for example from the patent document WO 98/14345. This invention concerns a method for determining several parameters of a seated person. The evaluation of said parameters can include the estimation of the size and/or the weight of said person.
However, various problem may occur if a seat provided with a heating element as mentioned above were also to be equipped with a separate sensor for detecting the presence of an occupant in said vehicle. Firstly, it can be noted that the heating element can only be adhered to a particular section of the surface of the seat cushion where the sensor is not mounted. This allows a relatively limited area for the mounting of the heating element. Furthermore, there is a risk that the occupant sensor surface will come into contact with adhesion glue which is used for the heating element. This may cause prebending of the sensor, which in turn may lead to deterioration of the detection signal from the occupant sensor. Also, such an known arrangement would comprise a large number of components, which would lead to high costs for the seat. Furthermore, such an arrangement would imply a complex and costly assembly of the seat, which would also lead to higher costs. Also, there is a risk that the operation of the two components, i.e. the heating element and the occupant sensor, would be interfered since the components are mounted very close to each other. Also, such an assembly of a heating element and an occupant sensor would imply a risk that the sensor would slide out of position during its lifetime. Finally, such an assembly of a heating element and an occupant sensor would imply a risk that the heating element would interfere with the operation of the occupant sensor, i.e. the sensor may supply erroneous signals if is is heated.
Consquently, there are various problems involved when trying to combine these two components within the same vehicle seat.